a. Field of the Invention
This invention relates to coating compositions which, when applied to substrate surfaces and exposed to actinic radiation in air, can form a crosslinked and cured film having excellent abrasion resistance, surface smoothness, thermal resistance, chemical resistance, durability, weather resistance and adhesion to the substrate, and to a process for producing abrasion-resistant synthetic resin molded articles by using such a coating composition.
b. Description of the Prior Art
Synthetic resin molded articles formed of polymethyl methacrylate resin, polymethacrylimide resin, polycarbonate resins, polystyrene resin and AS resin have lighter weight and higher impact resistance than glass products. Moreover, they have high transparency and can be shaped easily. Owing to these various advantages, they have recently come to be used in a wide variety of applications including automobile plastic parts.
On the other hand, the surfaces of such synthetic resin molded articles do not have sufficient abrasion resistance, so that they are subject to damage as a result of contact with other harder objects, friction, scratching and the like. Since any damage caused to the surfaces of such synthetic resin molded articles diminishes their commercial value significantly and/or makes them unusable in a short period of time, it is strongly desired to improve the abrasion resistance of their surfaces. Moreover, where they are used as automobile parts, their weather resistance is also regarded as essential.
In order to overcome the above-described disadvantages of synthetic resin molded articles, various attempts have heretofore been made. For example, U.S. Pat. No. 4,006,271 discloses a method for improving the abrasion resistance of synthetic resin molded articles wherein a coating material comprising a partially hydrolyzed and condensed product of a silane mixture composed chiefly of an alkyltrialkoxysilane and colloidal silica is applied to the surface of a synthetic resin molded article and then heated to form a crosslinked and cured film. Although a high degree of abrasion resistance can be achieved by this method, the resulting film tends to have insufficient adhesion to the surface of the molded article. In order to enhance this adhesion, it is necessary to use an acrylic polymer or a silicone as a primer. This is disadvantageous in that a complicated treating procedure is required. Moreover, the curing time is so long that economic losses and low productivity result.
In order to overcome these disadvantages, U.S. Pat. No. 4,348,462 discloses a method of making abrasion-resistant synthetic resin molded articles wherein an ultraviolet-curing coating material comprising colloidal silica, an alkoxysilane having a methacryloyl or glycidyl functional group, and a non-silyl acrylate is applied to the surface of a synthetic resin molded article and then exposed to ultraviolet light. In addition, U.S. Pat. No. 4,491,508 discloses the use of a coating composition comprising colloidal silica, a hydrolyzate of a silyl acrylate, a polyfunctional acrylate and a photopolymerization initiator and containing essentially no organic solvent. These methods involve the use of ultraviolet light for curing purposes and have the advantage that the long curing time which has heretofore been encountered with silicone-based coating films can be reduced markedly. Moreover, they are highly effective in improving the abrasion resistance of synthetic resin molded articles.
Of the above-described methods, however, the former one can enhance productivity, but has the disadvantage that the durability and weather resistance of the resulting cured film are less than satisfactory. Moreover, a complicated treating procedure is still required because the surfaces of synthetic resin molded articles must be undercoated with a primer composition prior to application of the coating composition. The latter method, which uses essentially no organic solvent, the coating film formed by applying the coating composition to a synthetic resin molded article has poor surface smoothness and tends to develop surface defects such as cissing and pinholes. Moreover, the properties of the cured film are reduced after being subjected to a hot water resistance test, a thermal cycling test or a weather resistance test. Furthermore, the above-described prior-art methods have the additional disadvantage that, owing to the properties of the monomers used, the coating composition must be cured in an atmosphere of nitrogen and cannot be cured in air.